1. Field of the Invention
The present invention relates to a nanoparticle phosphor and a method for manufacturing the same, a semiconductor nanoparticle phosphor and a light emitting element containing a semiconductor nanoparticle phosphor, a wavelength converter and a light emitting device.
2. Description of the Background Art
<First Background Art>
It has been known that when an average particle size of crystal particles composed of a compound semiconductor is made to be approximately the Bohr radius, a quantum size effect is exhibited. The quantum size effect refers to the property of electrons in a substance, when made to be approximately the Bohr radius, can no longer move freely, and the energy of the electrons in such a state is not arbitrary and only a specific value can be obtained. It has been proposed that a nanoparticle composed of a compound semiconductor (hereinafter, simply referred to as “nanoparticle”) is used as a phosphor utilizing the quantum size effect. For example, this nanoparticle has a particle size of around the Bohr radius, and has a property that as the size of the particle becomes smaller, a wavelength of light generated is shifted to a shorter wavelength side. This nanoparticle also has a property that as the size distribution of the particle becomes narrower, the full width at half maximum of the fluorescence spectrum of the particle becomes narrower, and color purity becomes higher (quantum size effect). For this reason, when two or more kinds of nanoparticles (phosphors) each having a different particle size are used, if the particle size distribution of each nanoparticle is narrow, it is possible to create various fluorescent colors.
However, when the crystallinity of the nanoparticle or the surface state of the nanoparticle is not good, an exciton may undergo non-radiative relaxation (inactivation). When an exciton undergoes non-radiative relaxation, the light emission property of the nanoparticle is degraded. Accordingly, it is required to suppress generation of non-radiative relaxation of an exciton.
It is considered that the energy level at which non-radiative relaxation of an exciton is generated mainly exists at a crystal defect site on the surface of the nanoparticle. For this reason, as a method for suppressing the generation of non-radiative relaxation of an exciton, it has been proposed that the surface of a core particle composed of a compound semiconductor is covered with a semiconductor material having a band gap energy greater than that of the compound semiconductor and having no band gap in a forbidden band of the compound semiconductor. In Japanese Patent Laying-Open No. 2004-51863, there has been proposed that a modifying group having a specified chemical structure is bound to the surface of the nanoparticle.
<Second Background Art>
It has been known that when a semiconductor nanoparticle is reduced to a size approximately as small as an exciton Bohr radius, a quantum size effect is exhibited. The quantum size effect refers to the property of electrons in a substance, when reduced in size, can no longer move freely, and the energy of the electrons is not arbitrary and only a specific value can be obtained. It has been known that, by changing the size of a semiconductor nanoparticle confining electrons, the energy state of the electrons is also changed, and as a dimension becomes smaller, the wavelength of light generated from the semiconductor nanoparticle becomes shorter as the size becomes smaller. The semiconductor nanoparticle exhibiting such a quantum size effect has drawn attention for utility as a phosphor, and research thereof has progressed.
In the fluorescence mechanism of a semiconductor nanoparticle phosphor, there is entirely another fluorescence mechanism by which fluorescence is considered to be generated from an energy level existing in a forbidden band of an energy level in the interior of the semiconductor nanoparticle, in addition to band gap fluorescence exhibited by the interior of the semiconductor nanoparticle. The former is fluorescence, the wavelength of which can be controlled by the particle size of the semiconductor nanoparticle. On the other hand, it is considered that the latter energy level generating fluorescence exists mainly at the surface site of the semiconductor nanoparticle. The energy level becomes a phenomenon of inhibiting the nature of the semiconductor nanoparticle having a narrow full width at half maximum, and it has been considered to be a problem to be solved.
In order to solve this problem, there has been proposed a technique of suppressing degradation in light emission efficiency due to a defect on a semiconductor nanoparticle surface, by forming, on the surface of the semiconductor nanoparticle, a covering layer composed of a substance having a band gap greater than a band gap possessed by the semiconductor nanoparticle and having no band gap in a forbidden band of the semiconductor nanoparticle.
Japanese Patent Laying-Open No. 2005-103746 discloses, as shown in FIG. 13, a technique of attaining high light emission property by arranging a layer 265 containing an electron donating group on the surface of a semiconductor nanoparticle 262, and further, imparting durability to an external factor by further covering the layer 265 with an organic substance layer 266.
<Third Background Art>
It has been known that when a semiconductor nanoparticle is reduced to a size approximately as small as an exciton Bohr radius, a quantum size effect is exhibited. The quantum size effect refers to the property of electrons in a substance, when reduced in size, can no longer move freely, and the energy of the electrons is not arbitrary and only a specific value can be obtained. It has been known that, by changing the size of a semiconductor nanoparticle confining electrons, the energy state of the electrons is also changed, and as a dimension becomes smaller, the wavelength of light generated from the semiconductor nanoparticle becomes shorter as the size becomes smaller. The semiconductor nanoparticle exhibiting such a quantum size effect has drawn attention for utility as a phosphor, and research thereof has progressed.
Since the semiconductor nanoparticle exhibiting the quantum size effect has a small average particle size of less than or equal to 100 nm, it has a great specific surface area. For this reason, when the surface of the semiconductor nanoparticle undergoes influence of oxidation or the like, the chemical stability of the whole semiconductor nanoparticle is damaged, which greatly influences the light emission intensity of a phosphor. Since the semiconductor nanoparticle has a high surface activity, it easily aggregates in a solution.
Then, in order to improve the chemical stability of the semiconductor nanoparticle and to prevent aggregation of the semiconductor nanoparticles, there has been proposed a technique of protecting the semiconductor nanoparticle by binding, for example, the surface modifying molecule such as trioctylphosphine oxide (TOPO) to the surface of the semiconductor nanoparticle.
When a phosphor composed of the semiconductor nanoparticle is used in a wavelength converter of a light emitting device, it is generally required to disperse the phosphor in a solid layer of a resin or the like. However, when the semiconductor nanoparticle having a surface modifying molecule contacts with a resin, the surface state of the semiconductor nanoparticle is changed, and aggregation is generated. For this reason, there is a problem that the light emission efficiency of the phosphor containing the semiconductor nanoparticle is degraded.
Then, International Publication WO 2011/081037 has proposed a technique of covering the surface of a semiconductor nanoparticle with a hydrolysate of metal alkoxide.